1. Field of the Invention
The present invention relates to a portable semiconductor data storage device having a single bit error correction function, and more particularly, to an improvement in the reliability of the data storage device.
2. Description of the Related Art
FIG. 7 shows a conventional portable semiconductor storage device. An input/output buffer 7 is connected to a voltage control circuit 30, and a group of memory ICs 1 for storing data is connected to the input/output buffer 7. The group of memory ICs 1 consists of eight least significant byte storing memories 2a to 2h for storing the least significant bytes, and eight most significant byte storing memories 3a to 3h for storing the most significant bytes. An address bus 9, a control bus 10 and a data bus 14 are connected to the input/output buffer 7. The control bus 10 includes an output enable signal line OE, a write enable signal line WE, and card enable signal lines CE1 and CE2. The input/output buffer 7 works as an address buffer, data bus buffer and a control bus buffer. Practically, the input/output buffer 7 can operate in accordance with the following function table.
__________________________________________________________________________ Operation Mode CE2 CE1 A0 D15-D8 D7-D0 __________________________________________________________________________ Standby Mode H H X High Impedance High Impedance The least H L L High Impedance Even Byte significant byte control The most H L H High Impedance Odd Byte significant byte control Word Control L L X Odd Byte Even Byte __________________________________________________________________________
In the above table, A0 indicates the least significant bit of the address bus 9, and D15-D0 denotes the individual bits of the data bus 14. That is, the input/output buffer 7 controls the least and the most significant byte and the word (the least significant byte+the most significant byte) by means of the card enable signal lines CE1 and CE2 and the least significant bit A0 of the address bus 9.
The operation of the above portable semiconductor data storing device will be described below. When the storage device shown in FIG. 7 is connected to a terminal machine (not shown), power is supplied from the terminal machine to a power input line 34. When the voltage on the power input line 34 has reached a predetermined threshold value, the voltage control circuit 30 connects the power input line 34 to an internal power line 35 and sends out a "H" level protect signal to the input/output buffer 7 to enable the input/output buffer 7. In this state, the terminal can access to the memory ICs in each of the modes listed on the function table.
On the other hand, when the power supply from the terminal machine stops and the voltage on the power input line 34 thereby drops to the predetermined threshold value or less, such as while the data storage device is being carried from place to another, the voltage control circuit 30 is disconnected, and sends out a "L" level protect signal to the input/output buffer 7. Consequently, all the chip select signals CSL0 to CSL7 and CSH0 to CSH7 output from the input/output buffer 7 to the least and the most significant byte storing memories 2a to 2h and 3a to 3h rise to the "H" level, and these memories 2a to 2h and 3a to 3h are thereby disabled. In that case, power is supplied from a cell 33 incorporated in the data storage device to the internal power line 35 through a reverse-charge preventing diode 31 and a limiting resistor 32 to back up the data stored in the memories 2a to 2h and 3a to 3h.
Soft errors, i.e., non-fatal recoverable random failures, in the memory ICs of the portable semiconductor data storage device, will be described. Generally, soft errors are caused in the semiconductor memories because of alpha rays, hot carriers, and other, random, failures. Soft errors normally occur in a single bit in the total storage areas of the memory ICs. Assuming that the soft error rate is 50 to 300 FIT/piece, the error rate of a portable semiconductor data storage device including 16 memory ICs is 800 to 4800 FIT. In the case of a system including, for example, 1000 portable semiconductor data storage devices, soft errors occur in 8 to 48 storage devices in 10.sup.4 hours (about in 1.14 years). FIT is the unit which indicates the failure rate. 1 FIT is 1.times.10.sup.-9 failures/operating hour.
Thus, generation of soft errors reduces the reliability of the conventional portable semiconductor data storage device and may lead to a failure in system operation.